During each cardiac cycle, the natural heart valves selectively open to allow blood to flow through them and then close to block blood flow. During systole, the mitral and tricuspid valves close to prevent reverse blood flow from the ventricles to the atria. At the same time, the aortic and pulmonary valves open to allow blood flow into the aorta and pulmonary arteries. Conversely, during diastole, the aortic and pulmonary valves close to prevent reverse blood flow from the aorta and pulmonary arteries into the ventricles, and the mitral and tricuspid valves open to allow blood flow into the ventricles. The cardiac valves open and close passively in response to blood pressure changes operating against the valve leaflet structure. Their valve leaflets close when forward pressure gradient reverses and urges blood flow backward and open when forward pressure gradient urges blood flow forward.
In certain individuals, the performance of a natural heart valve is compromised due to a birth defect or becomes compromised due to various disease processes. Surgical repair or replacement of the natural heart valve is considered when the natural heart valve is impaired to an extent such that normal cardiac function cannot be maintained. The natural heart valve can be replaced by homograft valves obtained from the same species (e.g., human donor heart valves), heterograft valves acquired from different species, and prosthetic mechanical heart valves.
The present invention is directed to improvements in prosthetic mechanical heart valves. Modern implantable mechanical heart valves are typically formed of a relatively rigid, generally annular valve body defining a blood flow orifice and an annular valve seat and one or more occluders that are movable between a closed, seated position in the annular valve seat and an open position at an angle to the valve body axis. These components of mechanical heart valves are made of blood compatible, non-thrombogenic materials, i.e., pyrolytic carbon and titanium. A bio-compatible, fabric sewing ring is typically provided around the exterior of the valve body to provide an attachment site for suturing the valve prosthesis into a prepared valve annulus. The occluder(s) is retained and a prescribed range of motion is defined by a cooperating hinge mechanism or other restraining mechanism. Such prosthetic heart valves function essentially as check valves in which the occluder(s) responds to changes in the relative blood pressure in the forward and reverse directions as described above and move between their open and closed positions.
A wide variety of mechanical heart valve designs have been proposed and/or utilized in the past. For example, an early clinically used mechanical heart valve employed a spherical ball moving into and out of engagement with an annular seat within a cage in response to the normal pumping action of the heart. Other clinically used heart valve prostheses have employed occluders in the form of a circular disc that pivots open and closed in response to blood pressure changes while being restrained by cooperative structure of the valve body.
A further clinically used bi-leaflet heart valve prosthesis employs a pair of semi-circular or semi-elliptical plates or leaflets that hinge open and closed together. Such bi-leaflet heart valves are typically entirely formed with a pyrolytic carbon or with pyrolytic carbon coating on all exterior surfaces of the valve body and leaflets. A typical method of coating pyrolytic carbon onto a valve substrate is disclosed in U.S. Pat. No. 3,526,005. The pyrolytic carbon coating provides wear resistant surface, and provides insurance against thrombus formation on such surfaces.
Bi-leaflet heart valves generally utilize pivot or hinge mechanisms to guide and control the motion of the leaflets between the seated, closed position and the open position. In such design configurations, two mirror image leaflets are typically disposed in opposed or mirror image relation to one another. Upon closure, each valve leaflet occludes or covers half of the annular valve orifice or valve annulus. Generally, each leaflet is designed with roughly semi-circular shape and has a rounded exterior margin and peripheral edge which engages an inner seat surface of the valve body to provide a peripheral seal, and an inner, diametrically extending edge and adjacent margins adapted to abut against the counterpart edges and margins on the other leaflet. Each leaflet can rotate about an axis defined by a pair of opposed hinge pivot points in opposed hinge recesses that are offset from the central axis of the valve annulus. The leaflets are typically flat, but curved or elliptical leaflets have been proposed.
Such mechanical heart valves are typically designed in somewhat differing profile configurations for replacement of differing impaired natural heart valves. However, the basic in vivo operating principle is similar regardless of configuration. Using an aortic valve as an example, when blood pressure rises in response to left ventricle contraction or systole in each cardiac cycle, the leaflets of such a valve pivot from a closed position to an open position to permit blood flow past the leaflets. When the left ventricle contraction is complete, blood tends to flow in the opposite direction in diastole in response to the back pressure. The back pressure causes the aortic valve leaflets to close in order to maintain arterial pressure in the arterial system.
The most widely accepted type of bi-leaflet heart valve presently used mounts its leaflets for pivoting movement by means of a pair of rounded ears extending radially outwardly from opposed edges of the leaflets to fit within rounded hinge recesses in opposed flat surfaces of the valve body side wall. Such bi-leaflet valves are exemplified by the mitral valve depicted in U.S. Pat. No. 4,276,658 and the aortic heart valve depicted in U.S. Pat. No. 5,178,632, both incorporated herein by reference.
The leaflet ears are received within curved hinge recesses extending radially into opposed flat surfaces of thickened wall sections inside the annulus of the generally cylindrical or annular valve body. Each hinge recess is designed in at least one respect to match the shape of the leaflet ear and is bounded by sets of leaflet stop surfaces angled to define the extreme open and closed leaflet positions. In other words, where the ear is formed as a portion of a circle having a given radius, the counterpart hinge recess is formed as a semi-circle having a slightly greater radius. An inverse arrangement of the ear and recess hinge mechanism is depicted in U.S. Pat. No. 5,354,330, incorporated herein by reference, whereby the leaflet ear is replaced by a leaflet recess, and the hinge recess is replaced by a complementary shaped hinge boss.
To achieve the pivoting mechanism, the mating surfaces of the ears and recesses are precisely machined so as to provide a small but definite working clearance for the ears to pivot about the necked down pivot surface and be retained within the hinge recesses. During valve assembly, the annular valve body is deformed or distended so that the leaflet ears may be inserted into the respective hinge recesses. Each manufactured heart valve is then lab tested "dry" to ensure that the leaflets are held tightly enough to be secure against falling out of their hinge recesses, but are not so tightly engaged so as to create a binding or restricted valve action.
The range of leaflet motion is typically controlled by pins or ramps or opposed side stops of the hinge recesses or by hinge bosses in the valve body. In one format described in the above-incorporated '632 patent, the hinge recess is generally spherical and bounded by open and closed stop surfaces of a stop member projecting into the recess. In the other formats depicted in the above-mentioned incorporated, '658 and '046 patents, each hinge recess has an elongated "bow-tie" or "butterfly" appearance created by the inward angulation of opposed side edges extending from inflow and outflow end edges and meeting at opposite disposed, necked down, pivot points or surfaces intermediate the end edges.
A great deal of effort has been devoted to controlling the range of movement and the acceleration of the leaflets between the open and closed positions to both control noise and decrease wear or the possibility of leaflet fracture. In the past, bi-leaflet valves were known to be noisy, in the sense that patients could frequently hear the seating of the valve leaflet peripheral edges against the valve seats when they closed. It is desirable for patient comfort to provide a bi-leaflet design that minimizes the distraction of leaflet seating noise.
It is also known that blood cells are extremely fragile and delicate and can be damaged and/or destroyed when trapped in the valve seat during closure of the valve leaflet or in the wiping area of the valve leaflet ears and hinge recesses or between the leaflet ears and the open and closed stop surfaces. The wiping areas of the hinge recesses have the highest potential of thrombus formation and emboli entrapment which can accumulate therein, impair the movement of the valve leaflets, and result in valve failure requiring surgical intervention. The close tolerances and resulting narrow gaps between the leaflet ears and the hinge recess surfaces contribute to this problem. During the open leaflet phase, blood barely flows through the recess under very low forward pressure gradient. During the closed leaflet phase, blood flow in the hinge recess region is, of course, stopped. Any existing thrombus and/or entrapped emboli in the hinge recess can restrict leaflet motion, which in turn can further enhance thrombus formation and trapping more emboli. To this time, no design has been successful in eradicating this problem. Consequently, patients receiving current bi-leaflet mechanical heart valves are prescribed continuous blood anticoagulation therapy to prevent thrombus formation and thromboemboli.
In this regard, in prior art bi-leaflet prosthetic heart valves described above employing the generally concave hinge recesses formed in planar surfaces to receive generally convex leaflet ears, the side walls that stop movement of the leaflet ears and define the leaflet open and closed positions are designed to be orthogonally cut into the planar surfaces, resulting in relatively abrupt or "sharp" edges with the planar surface. This configuration allows relatively large areas of contact between the sides of the valve ears and the recess side walls in the open and closed leaflet positions. The hinge recess end boundaries or edges extending between the ends of the side walls in the inflow and outflow directions are also designed with relatively sharp transitions at the planar surfaces. Relatively sharp corners are created at the junctions of the ends of the side walls and the ends of the inflow and outflow recess end edges. These sharp edges and corners create unnecessary blood flow stagnation regions and blood flow separations. The sharp edges and corners are also high mechanical stress concentration sites and may also be potential structural failure initiation sites.
In spite of significant advances which have taken place in the construction of mechanical heart valves, there is still room for significant improvements therein to minimize flow stagnation regions inside the valve annulus, especially in the hinge recesses, which may lead to thrombus formation and cause the failure of a prosthetic valve, to reduce impact force upon valve closure, and to eliminate flow turbulence in or near valve annulus.
This invention is therefore directed to improving the current hinge mechanisms employed in bi-leaflet prosthetic mechanical heart valves to increase blood washing of the hinge recess, improve valve hemodynamic performance, and to reduce leaflet impact force, valve failure potential, and the need for long term anticoagulant therapy.